Shuttle-engines Test Scheduled For Today

WASHINGTON — The space shuttle Discovery's highly advanced main engines, which are set to be tested this morning, have been plagued with problems since the early design phases.

Once described as the shuttle's ''Achilles' heel'' by J.R. Thompson, the head of the Marshall Space Flight Center in Huntsville, Ala., the engines have undergone 39 changes that are designed to reduce their volatility.

After the Challenger accident, a House Science and Technology Committee investigation report expressed concern about turbine blade cracking problems, burned bearings and an inadequate amount of shuttle engine testing.

''We are impressed by the sophistication and performance of the main engine, but are concerned that it may have inadequate safety margins to ensure continued safe operations,'' the committee said.

Since the accident, engine parts have been redesigned and inspection and maintenance rules strengthened. Certain parts must be replaced after every flight or every few flights to increase the safety margin. The added precautions, considered crucial, have shortened the engines' life and increased their cost and processing time.

To test the engines' safety since the accident, sample ones have been fired for the equivalent of about 50 shuttle missions.

''We have been testing that engine very rigorously, aggressively, pushing the margins to the limits,'' Thompson said recently. ''In the last month alone we have run three or four tests more than twice the normal flight duration'' of about eight minutes.

Before the first shuttle flight, test data indicated that the engines would be cut off on the launch pad before booster ignition one time in 10 launches and that an engine would fail in flight one time in 20. The 24 flights before Challenger included two launch pad aborts and one in-flight shutdown, just matching the prediction.

Thompson and others say the improvements and extensive testing should make those odds better, but they would not give new estimates.

Since the development of the engines in the 1970s, ''we have learned how to build it, how to design it, and how to care for it,'' said Bob Morris, manager of the shuttle engine flight management office at the Marshall center.

After today's test, NASA managers will decide about fixing a leak in Discovery's aft orbital steering system while the shuttle is on the launch pad.

Engineers expect to cut a small hole in the right rear corner of the cargo bay to get at the leak and plug it with a clamshell device. The work is expected to require up to two weeks, and would set back the launch until late September or early October.

Any problems during the test-firing itself could set back the date even more.